Advice needed - Nifty fifty or Flash

I am a father of two young kids and just bought a new Canon T3i and loving it. I mainly take photos of kids and somewhat equal amount of photos outside as well. Currently I have 18-55 Kit and 55-250 IS lenses.

I am planning to get ONE more accessory. Currently I am looking at either 50mm prime 1.8 or Canon 270EX II (or YN-560) flash. Not both.